Completing an oil or gas well includes a cementing procedure that bonds one or more well casings lining a wellbore to a surrounding subterranean formation and between each other. In the vast majority of cases, as the well deepens, new casing strings are needed and cemented in place. Many other factors can also indicate the need for multiple concentric or overlapping casing strings, such as compliance with environmental and safety policies. The cement between these casing strings generally prevents the presence or movement of fluids within the annular spaces defined between overlapping casings and between the casing and the wellbore wall. In some wells, the cementing process extends from total depth to surface, while in others the cement is present only between certain depths.
Of particular importance is the determination of the presence of fluid flow paths in the annular regions defined between overlapping casings and between the casing and the wellbore wall due to an absence of cement (or cement bond) at or between certain depths. The identification and characterization of these flow paths is particularly critical in the case of plug and abandonment operations, especially in deep water applications.
Sonic tools or ultrasonic tools are typically used in the industry to evaluate the cement bonding to both the formation and the casing and hence infer potential annular flow paths (i.e., leaks, channels, gaps, etc.). The evaluation in most of these cases uses raw data from only one system of sensors to infer fluid flow parameters through indirect methods. There is no current workflow method that integrates the various measurements obtained into a borehole model. Current methods are also typically used for the characterization of the first casing-cement bond, thereby precluding the evaluation of any subsequent interfaces in the case of multiple casing strings extended within the well.